1. Field of the Invention
The present invention relates to a method for restraining inter-cell interference in a mobile communication system. More particularly, the present invention relates to a hybrid method of fast dynamic selection of a fractional frequency reuse technology and Macro Diversity technology in a cell edge, and the method is applied in an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system downlink.
2. Description of the Prior Art
The Orthogonal Frequency Division Multiple Access (OFDMA) technology is used with the 4th generation (4G) mobile communication technology. The standards for 4G technologies are developed and regulated by several primary organizations, such as IEEE 802.16m, 3GPP LTE-Advanced, and 3GPP2 UMB+, which are all concerned with air interface technologies based on OFDMA technology.
In the OFDMA system, the time-frequency two-dimensional electric waves are composed of a Orthogonal Frequency Division Multiplex (OFDM) signal in the time domain and a frequency subchannel in the frequency domain. Each frequency subchannel is composed of a plurality of different subcarriers. In an OFDM signal time interval, each user in the cell uses a orthogonal frequency subchannel, therefore, the OFDMA system is free from intra-cell interference, which is an important character of the OFDMA system. When different cells or users use the same frequency subchannel for transmitting messages in the same time interval, inter-cell co-channel interference (also called inter-cell interference) occurs. Thus, the link quality of the cell edge user degrades and the data throughput decreases, which are serious problems in OFDMA systems.
In the future, the 4G mobile communication systems, such as the IEEE 802.16m, 3GPP LTE-Advanced, and 3GPP2 UMB+, all utilize inter-cell interference coordination technology to solve the problem of inter-cell interference. The inter-cell interference coordination technology is configured to coordinate the frequency, time, and/or emitting power between the neighbor cells in advance, to avoid or decrease inter-cell interference. Presently, many methods have been developed to reduce inter-cell interference, such as the partial frequency reuse, fractional frequency reuse, soft frequency reuse, and inverted frequency reuse technologies. The fractional frequency reuse (FFR) technology is considered to have the greatest potential for development and is widely used. Presently, pre-4G technologies, such as the 3GPP2 UMB and Mobile WiMAX (IEEE 802.16e), uses FFR technology to combat the problem of inter-cell interference.
FFR technology is a kind of frequency-domain interference coordination technology, which applies a frequency reuse factor (FRF) larger than 1 for planning the frequency in the cell edge region to reduce inter-cell interference. Thus the link quality is considered to be improved, and the data throughput is considered to be increased. However, FFR technology applies the FRF equal to 1 (reuse-1 or FRF=1) in the cell center region to maintain superior system capacity. The use of FRF equal to one creates an inefficient use of cellular system resources.
A mobile communication network comprises a base station controlling one or more cells (or sectors). Usually, a base station controls three cells (sectors). FIG. 1A shows a base station 10 controlling three cells. FIGS. 1A and 1B are conventional frequency resource allocation diagram for realizing the FFR technology in the mobile communication network base station 10. The useful frequency shown in the system in FIGS. 1A and 1B is divided into center subband F1 12 and edge subband F3 11, in which the edge subband F3 11 is divided into three orthogonal subbands F3A, F3B, and F3C, that is, the useful frequency comprises four orthogonal subbands.
Referring to FIGS. 1A and 1B, the center subband F1 12 is adapted for an FRR equal to 1 (reuse-1) reuse method, which means all cells can use the subband. The edge subband F3 11 is adapted for a FRF equal to 3 (reuse-3 or FRF=3) reuse method, and the three subbands F3A, F3B, and F3C are adapted for the cell A13, cell B14 and cell C 15 of the base station 10 respectively. Taking cell A 13 as an example, the center subband FT 12 of the cell A 13 is allocated to the neighbor users around the center of cell, and the spectral efficiency of the subband is the highest one. On the other hand, the edge subband F3A of the cell A 13 is first allocated to the cell edge user. At this time, the reuse-3 reuse method is applied, and the link quality of the edge user is improved.
For general purposes, the mobile communication network is assumed to have a base station 10 controlling three cells (sectors).
FIG. 2 illustrates the conventional FFR technology operation flow chart for application in a mobile communication network, comprising the steps as follows:
Step 1 is executed so that the User Equipment (UE) is configured to measure the link signal quality and report the result to the serving cell (or serving sector) (201), in which the Signal Quality Index (SQI) can be a wideband average signal to interference plus noise ratio (SINR).
Step 2 is executed so that a detector in the serving cell determines whether the UE is a cell center user or a cell edge user by determining whether the signal quality is smaller than a threshold level (202).
Step 3 is executed to identify the UE as a cell center user once the wideband average SINR is not smaller than the threshold level (203).
Step 4 is executed so that when the UE retrieves the first transmission priority from a scheduler and the UE is a cell center user, the system then allocates the frequency subchannel of the center subband (i.e. F1) of the serving cell to the user, and processes the transmission with the reuse-1 method (204).
Step 5 is executed to identify the UE as a cell edge user once the wideband average SINR is smaller than the threshold level (205).
Step 6 is executed so that the system allocates the frequency subchannel of the edge subband (i.e. F3A, F3B, or F3C) of the serving cell to the user when the UE is a cell edge user, and processes the transmission with the reuse-3 method (206).
According to the technical requirements of 4G technology issued by the International Telecommunication Union (ITU), the cell edge data rate is an important performance index. However, it is generally believed that the system capacity and the cell edge capacity have an inverse relationship. Presently, a OFDMA mobile communication system that applies the FFR technology to combat inter-cell interference is able to improve the cell edge capacity (or the cell edge data rate) but must then sacrifice significant system capacity. Thus, developing a method for restraining inter-cell interference with better efficiency as well as obtaining a better balance between the system capacity and the cell edge capacity becomes an important and challenging issue for mobile communication systems based on the OFDMA technology.
The inventor made improvements over the aforementioned drawbacks of the conventional products, and develops the present invention of method for restraining inter-cell interference in a mobile communication system.